A Schmitt trigger is a comparator circuit with hysteresis. The circuit is named a “trigger” because the output retains its value until the input passes upward through a threshold voltage, triggering a change. The circuit is a hysteresis circuit because it uses negative feedback to prevent switching back until the input passes through a lower threshold voltage.
Schmitt trigger devices can be used in open loop configurations to remove noise from signals used in digital circuits, for example, to eliminate signal noise caused by mechanical switch bounce. Closed loop configurations can also use Schmitt trigger devices, such as oscillators, used in function generators and switching power supplies.
Schmitt triggers can be non-inverting or inverting. In non-inverting Schmitt triggers, the output transitions to a higher value when the input increases past the threshold voltage. In inverting Schmitt triggers, the output transitions to a lower value when the input increases past the threshold voltage.
FIGS. 1A-1B illustrate a schematic behavior of a prior art non-inverting Schmitt trigger circuit. FIG. 1A shows a Schmitt trigger circuit 100, including an operational amplifier 110 and resistors R1 and R2. The positive feedback provided by these resistors creates a hysteresis controlled by the proportion between R1 and R2. Since the operational amplifier 110 has a differential input, the inverting input is grounded to make the reference point zero volts.
FIG. 1B shows a response function of the non-inverting Schmitt trigger circuit 100, which shows the output voltage Vout as a function of the input voltage Vin. When the input voltage Vin is above the high threshold T or below the low threshold −T, the output voltage has the same sign as the circuit input voltage, e.g., the output voltage is M or −M, respectively. When the circuit input voltage is between the thresholds −T and T, the output voltage Vout depends on the last state, e.g., Vout is M if previously Vin>T and Vout is −M if previously Vin<−T. For inverting Schmitt triggers, the behavior is reverse.
A comparator circuit can compare two input voltages or currents and outputs a digital signal indicating which is larger. Thus a comparator circuit can include two analog inputs, e.g., Vin and Vref, and one binary digital output Vout. If Vin is greater than Vref, Vout is positive. If Vin is smaller than Vref, Vout is zero. A comparator circuit can be used in analog-to-digital converters (ADCs) and oscillators.
FIGS. 9A-9B illustrate a schematic behavior of a prior art comparator circuit. FIG. 9A shows a comparator circuit 900, including an operational amplifier 910 and optional resistor R. The differential input of the operational amplifier 910 can be used to provide the comparison between the input voltage Vin and the reference voltage Vref.
FIG. 9B shows a response function of the comparator circuit 900, which shows the output voltage Vout as a function of the input voltage Vin. When the input voltage Vin is larger than the reference voltage Vref, the output voltage is M. When the circuit input voltage is smaller than the reference voltage Vref, the output voltage Vout is zero.
The Schmitt trigger circuit and the comparator circuit are implemented using transistor based logic components, which can have issues related to the transistor devices. Therefore, there is a need for Schmitt trigger and the comparator circuits that can meet the design criteria for advanced devices.